1. Field of the Invention
The present invention relates to an image pickup apparatus employed in a device for automatically performing visual inspection of wiring patterns of a printed wiring board, and more particularly, it relates to improvement in an illumination system for illuminating the printed wiring board.
2. Description of the Prior Art
In general, a printed wiring board is formed by a laminated plate member covered by copper foil. The copper-covered laminated plate member is polished in advance to etching processing by passage through, e.g., a buffing roller. Such buffing operation is so performed that a photoresist film is effectively adhered to the copper-covered laminated plate member and the surface of the copper foil is activated, thereby to smoothly perform the etching processing. After the etching processing, the photoresist film is removed. Copper patterns thus formed have flaws caused by the buffing operation on the surfaces thereof, which flaws define strias substantially in the same direction with respect to the printed wiring board. The directions of such strias are not necessarily identical with respect to printed wiring boards of the same kind.
FIG. 1 is a partially fragmented perspective view of a printed wiring board. A printed wiring board 3 which comprises a copper pattern 1 having substantially V-shaped strias 2 is subjected to successive image pickup in defect inspection of wiring patterns by an image pickup apparatus. Illumination light 4 is irradiated on the surface of the printed wiring board 3, so that light 5 reflected by the surface of the printed wiring board 3 is received in image pickup means, e.g., a solid state image sensor (not shown) through a lens (not shown). The solid state image sensor, which serves as the image pickup means, outputs image pickup signals in response to the amount of received light. A substrate base portion 6 and the copper pattern 1 are discriminated from each other on the basis of the image pickup signals, whereby information is obtained as binary picture images. Thus, it is necessary to facilitate such discrimination in the signal processing system by making the copper pattern 1 as the object of image pickup sufficiently different in reflected light amount from the substrate base portion 6 as the background.
However, since the copper pattern 1 has the strias 2 on its surface, the illumination light 4 is diffusedly reflected by the strias 2. The extent of the diffused reflection depends on relation between the direction of irradiation of the illumination light 4 and the direction of the strias 2, and hence the reflected light on the copper pattern 1 and that on the substrate base portion 6 are not necessarily in fine contrast. In other words, the strias 2 appear in various directions even on printed wiring boards of the same kind, and hence the amount of light reflected in specific directions such as a regularly reflected direction and the direction of setting of the image pickup means is changed at considerable rate depending on the direction of the strias 2, assuming that the illumination light 4 is irradiated in a constant direction. Since the amount of reflected light from the substrate base portion 6 is constant in any desired portion and on printed wiring boards of the same kind, the contrast is changed depending on the direction of the strias 2, i.e., depending on the printed wiring boards. Such changes in contrast cannot be effectively absorbed if discriminativity in the signal processing system is not sufficiently highly increased, particularly when pattern data of printed wiring boards are to be compared in pixel unit with each other.
Improvement in discriminativity in the signal processing system leads to remarkable increase in cost. Therefore, the aforementioned changes in contrast have generally been covered by extremely high-powered illumination so that image pickup signals relating to copper patterns are sufficiently detected even if the amount of reflected light from the copper patterns is at the minimum value. Such high-powered illumination is generally performed by tungsten or halogen lamps. However, the high-powered illumination employing the tungsten or halogen lamps has the following disadvantages:
In the first place, there is a problem of heat generation, such that the lamp generates a large quantity of heat by the high-powered illumination. Since a signal processing circuit is arranged in the vicinity of the lamp, significant thermal influence is exerted not only on a solid state image sensor but on respective parts of the circuit, such that, e.g., the dark current of the solid state image sensor is increased by increase in the ambient temperature. Further, in order to prevent such thermal influence, arrangement of the mechanical portion, the circuitry portion and the like must be specifically designed or a large-scale mechanism is required for heat generation.
In the second place, the lamp itself has such a problem that it must be large-sized for performing high-powered illumination, leading to increase in volume of the entire device. Further, the lamp is mechanically weak, relatively undurable, considerably defective in stability of the luminous power and inconvenient to handle.